The present invention relates to a method for making optical elements such as lenses or prisms, and optical preforms of the optical elements, by press molding.
One example of a method for making such preforms in the prior art is disclosed in Japanese laid-open patent application (Tokkai Hei) 5-177725. This method uses injection molding of molten plastic pellets to make the optical preforms as explained below.
FIG. 17 shows a cross section of an injection molder used for making optical preforms in the prior art. Plastic pellets 51 are supplied into the hopper 50 and transferred to the heating cylinder 53. The plastic pellets are heated and melted by the heating cylinder 53 and the screw 54. Molten resin is injected from the tip nozzle 55 of the heating cylinder 53 into the mold 59, and reaches cavities 63 corresponding to an outer shape of an optical element through a sprue 60, runners 61 and gates 62. The temperature of the mold 59 is controlled at approximately softening temperature. In FIG. 17, numeral 52 is an injection cylinder, 56 is a fixing die plate, 57 is a moving die plate, and 58 is a closing cylinder.
The mold 59 is opened and a molded piece is taken out of the mold after the mold has been cooled. As seen in FIGS. 18A and 18B, the molded piece has a center sprue portion 64, and a plurality of runner portions 65 extend radially from the center sprue portion 64. At the tip of each runner portion 65, an optical preform 67 is connected via a gate portion 66. A plurality of optical performs 67 are obtained by separating them from the molded piece at the gates.
Optical preforms are pressed to make optical elements in such a way as explained below. FIG. 19 illustrates a section of a press mold and an optical element when the pressing is performed. Numeral 68 is an upper mold, 69 is a lower mold, and 70 is a drum mold. Numeral 71 is an optical element obtained by press molding, 72 is a part of press head having heating and pressing mechanisms, and 73 is a part of a press stage having a heating mechanism. FIG. 20 shows a process chart for pressing an optical preform to make an optical element using press molds mentioned above. In this chart, the temperature of the optical element is shown in (a), and the pressure given by the press head is shown in (b).
An optical preform made from an optical material (e.g., polycarbonate) by injection molding into a shape similar to an optical element is placed in a cavity defined by the upper, lower and drum molds 68, 69, 70. These molds are controlled at a temperature above the softening temperature and below the glass transition point. When the temperature of the optical preform goes up to a constant temperature between the softening temperature and the glass transition point as shown in (a) of FIG. 20, the press head 72 is forced downward such that the upper mold 68 gives the optical preform the pressure of approximately 100 kg/cm.sup.2 for predetermined time. Then the pressure is released and the molds are cooled down to the softening temperature. After that, the upper mold is released and the pressed optical preform, i.e., the optical element 71 is taken out.
In the manufacturing process of optical preforms by injection molding in the prior art, surface sink, jetting, weld line or residual stress should be suppressed by controlling the conditions of injection molding. However, residual stress and distortion in the gate portion are difficult to eliminate completely by controlling the conditions of injection molding. Therefore, the optical preforms made by injection molding in the prior art have residual stress and distortion partially. It is difficult to make an optical element having good optical performance using such an optical preform having residual stress and distortion in the gate portion.
In addition, cost reduction in the process of making optical preforms by injection molding is difficult, because the number of products obtained in one shot is limited. Moreover, there is a difficulty in manufacturing smaller optical elements as the gate diameter has a lower limit due to a fluidity of the optical material.
Molds for a injection molder have a tendency to have a complicated structure for gaining a good fluidity of the optical material, and it is very expensive due to a labor cost for designing and trial manufacture of a proper mold structure. In addition, the wasted portion of the optical material in the injection mold is rather large. These disadvantages in the injection mold cause an extremely high cost for manufacturing optical preforms in the prior art.